This invention relates to machine vision systems and more particularly to systems for counting objects on a moving conveyor belt or other elongated surface.
During mass production processes, conveyor belts are used extensively. Manufactured objects such as cans, pill bottles, small parts, and the like, are transported between production processes on a variety of conveyor belts. Often, conveyor belts are wider than the minimum width of a given object and, thus, objects may be transported down a conveyor belt in a somewhat staggered random arrangement with objects often dispersed along the conveyor belt in a variety of spacing and orientations with respect to each other.
Machine vision systems are capable of counting distinct shapes on a surface using a variety of advanced techniques including pattern recognition. In general, one or more images of the objects within a field of view are captured and analyzed, producing an object count. However, most machine vision systems rely on one camera (or at most a few cameras) with a limited field of view for acquiring images of objects. A conveyor is, by definition, a constantly changing picture within any field of view. When the conveyor belt carries a somewhat random spacing and arrangement of objects, it is very difficult to accurately count these objects, especially when images are captured at a relatively high rate. The combination of unpredictable arrangement of parts and relative speed of movement along the conveyor belt makes it hard to track these objects between image captures, even if the underlying objects are fixed to a conveyor belt by means of a vacuum belt system. In other words, it is possible for objects to pass out of the field of view between image captures (and be missed entirely), or to be double-counted in consecutive image captures
It is, therefore, an object of this invention to provide a system and method for accurately tracking and counting objects as they move down a continuously moving conveyor belt at a normal process speed. The placement of the objects on the belt should not affect counting accuracy. This system and method should further enable counting of stationary objects on an elongated surface covering several camera fields of view.
This invention overcomes the disadvantages of the prior art by providing a system and method for more accurately counting objects or parts on an elongated surface (a continuous moving conveyor belt, herein) having relative movement with respect to a camera with a limited field of view, thereby requiring multiple fields of view. The system acquires a plurality of images from the belt, representing a plurality of xe2x80x9cfields of viewxe2x80x9d as consecutive portions of the belt (with associated parts thereon) pass below the camera""s area of interest. Each of the fields of view includes flanking right and left overlap zones with respect to adjacent fields of view. The overlap zones are defined to be at least as wide as the maximum width of a part being counted. The captured fields of view are stored within the system and analyzed in succession based upon a set of rules. These rules determine whether or not a particular part within the subject field of view is to be counted. More specifically, the rules are applied within the overlap zones for each field of view to determine whether or not to count the part. In a preferred embodiment, in general the basic counting rules (subject to refinements to be described below) for each field of view a on belt moving in an xe2x80x9cupstreamxe2x80x9d (left, herein) to xe2x80x9cdownstreamxe2x80x9d (right, herein) direction are as follows:
1. If a part resides partially or fully in the upstream overlap zone, then do not count the part.
2. If a part resides partially or fully in the center section, then count the part.
3. If a part resides fully in the downstream overlap zone, then count the part.
4. If a part resides partially in the downstream overlap zone, then do not count the part.
According to one embodiment, the parts are typically held against the belt during movement using, for example, a vacuum belt. The inspection station includes a CCD camera, or equivalent, having a triggered electronic shutter or triggered strobe flash that sets a limited periodic interval for viewing the parts in each field of view. The trigger can be switched by an encoder operatively connected to the belt and arranged to activate the shutter or strobe periodically as the belt moves a predetermined incremental distance.
In order to account for inherent errors within the conveyor and imaging system, the overlap zones can further define uncertainty regions that are generally twice the maximum acceptable error in width. Based upon the position of the assigned xe2x80x9coriginxe2x80x9d of each part image, within a field of view, with respect to the uncertainty regions, a part is either considered for counting/non-counting, and/or assigned to a holdover set that includes the part""s location and whether it was counted. If a match to the part is found in the previous holdover set, then the part is counted if it was not already counted.